This invention relates generally to the production of phosphor screens for color picture tubes and particularly to a system and method for maintaining a constant light intensity-time multiple in a lighthouse used to produce such screens.
A color picture tube includes a screen composed of triads of different phosphors which emit different colored light when excited by electrons. Typically, the screen is composed of alternating stripes of phosphors which respectively emit red, green and blue light. Positioned between the screen and the electron gun from which the exciting electrons emanate is a color selection electrode, commonly called a shadow mask. The shadow mask assures that the electron beams excite phosphor stripes of the proper color.
During the production of the phosphor screen, the entire inside surface of the panel is coated with one of the phosphors mixed in photosensitive material. The shadow mask is then inserted into the panel and the assembly is placed onto a lighthouse which contains a light source. Light from the light source passes through the apertures in the shadow mask and exposes some of the phosphor. The shadow mask is then removed and the unexposed phosphor is washed away leaving only the exposed phosphor. This process is then repeated for the remaining two colors of phosphors.
Consistent quality of the screens is essential and therefore any changes in the intensity of the exposing light must be accommodated. The intensity of the light can vary due to either variations in the input power or because of degregation of the lamp. Additionally, when panels for various sizes of tubes are randomly placed onto the lighthouse the exposure must be changed. In either event, as the intensity of the lamp changes, the exposure time must be proportionately and inversely changed in order to maintain a constant intensity-time multiple. Additionally, a change in the shadow mask light transmission characteristics also require a change in the exposure time.
The instant invention is directed to a system for maintaining a constant light intensity-exposure time multiple to assure uniform exposure of the phosphors in color television screens.